1. Field of the Invention
The present invention relates to an optical disc for storing various information signals such as musical tone signals, video signals, data signals, etc., which can optically be read out for reproduction.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows one conventional read-only optical disc which stores various information signals such as musical tone signals, video signals, data signals, etc., which can optically be read out for reproduction. The optical disc, designated by the reference numeral 11, comprises a disc substrate 12 having a signal pattern 13 which has been formed as a pattern of pits and lands on one surface thereof according to an information signal, a thin reflective layer 14 deposited on the signal pattern 13, and a protective layer 15 deposited on the reflective layer 14 for protecting the same.
The disc substrate 12 is injection-molded of a light-transmissive synthetic resin such as polycarbonate, PMMA, or the like by an injection molding machine with a stamper which has a pattern of pits and lands based on the information signal to be recorded on the optical disc 11. Specifically, the light-transmissive synthetic resin is injected into a cavity in the injection molding machine, and the pattern of pits and lands on the stamper is transferred onto one surface of the cake of light-transmissive synthetic resin in the cavity, thereby forming the signal pattern 13 on the surface of the disc substrate 12.
According to another known optical disc fabrication process, a layer of ultraviolet-curing resin is coated on one surface of a flat glass substrate. Then, a stamper having a pattern of pits and lands according to an information signal to be recorded is pressed against the coated layer of ultraviolet-curing resin. While the stamper is being pressed against the coated layer of ultraviolet-curing resin, an ultraviolet radiation is applied to the layer of ultraviolet-curing resin through the glass substrate, thus curing the layer of ultraviolet-curing resin on the glass substrate. Thereafter, the stamper is detached from the cured layer of ultraviolet-curing resin. In this manner, the pattern of pits and lands on the stamper is replicated on the layer of ultraviolet-curing resin on the glass substrate, thus producing a disc substrate.
Generally, the former disc substrate injection-molded of light-transmissive synthetic resin is widely used because it is less costly to manufacture than the latter disc substrate with the layer of ultraviolet-curing resin.
The thin reflective layer 14 is deposited on the signal pattern 13 of the disc substrate 12 which may be fabricated according to either the former or the latter fabrication process. The reflective layer 14 may be made of Al, Au, or the like, but is most commonly made of Al due to its lower cost. Using an evaporation source of Al, the thin reflective layer 14 is formed by evaporation or sputtering over the signal pattern 13 on one surface of the disc substrate 12.
Subsequently, the protective layer 15 is deposited on the reflective layer 14 to protect the same. The protective layer 15 is made of ultraviolet-curing resin. Specifically, the ultraviolet-curing resin is dropped in an annular pattern on a radially inner region of the reflective layer 14 on the protective layer 15, and then the disc substrate 15 is rotated at high speed by a rotary actuator to cause the dropped ultraviolet-curing resin to spread radially outwardly as a thin layer under centrifugal forces. The radially outwardly spread thin layer serves as the protective layer 15.
The read-only optical disc, typically a compact disc, an optical video disc, or the like, is manufactured in the manner described above.
One problem with the conventional optical disc 11 is as follows: Due to aging of the optical disc 11, air tends to find its way into the optical disc from between the disc substrate 12 and the protective layer 15, and to oxidize the aluminum of the reflective layer 14 when held in contact therewith. When the aluminum of the reflective layer 14 is oxidized, it turns to aluminum oxide which is transparent, and the reflective layer 14 can no longer perform its function.
In addition, since the reflective layer 14 of metallic material and the protective layer 15 of ultraviolet-curing resin do not adhere to each other with high bonding strength, the protective layer 15 is liable to peel easily from the reflective layer 14.